1. Field of the Invention
The present invention relates to an image sensor and an image capturing apparatus using the same.
2. Description of the Related Art
There has been proposed an image capturing apparatus using, as a method of detecting the focus state of an imaging lens, a pupil division phase-difference method (imaging plane phase-difference method) using a two-dimensional image sensor having a microlens formed on each pixel.
In an optical system, such as a camera lens unit, the entrance pupil is the optical image of the aperture stop viewed from the front of the lens. The corresponding image of the aperture viewed from the rear of the lens is known as the exit pupil. The image of the aperture stop will be in focus at a particular distance beyond the camera lens unit and this distance is known as the pupil distance of the imaging lens. Modern image sensors often include microlens arrays around the pixels in order to improve light capture efficiency. As a consequence of this, the image sensor is only able to accept light from a limited range of angles and each photodiode, and hence the image sensor, has an entrance pupil that is an image of the openings to the photodiodes viewed through the microlenses. The entrance pupil distance of the image sensor is a distance at which those openings are in focus through the microlenses.
U.S. Pat. No. 4,410,804 discloses an image capturing apparatus using a two-dimensional image sensor having one microlens and a plurality of divided photo-electric conversion portions formed for one pixel. Divided photo-electric conversion portions are configured to receive light from different areas of the exit pupil of the imaging lens through one microlens, thereby performing pupil division. This apparatus performs focus detection by obtaining an image shift amount from the respective signals received by these divided photo-electric conversion portions, and acquires an imaging signal by adding the signals received by the divided photo-electric conversion portions. This patent literature also discloses that the apparatus can obtain a stereoscopic image by separately displaying the parallax signals, received by the laterally divided photo-electric conversion portions on each pixel, for the right and left eyes. Japanese Patent Laid-Open No. 2000-156823 discloses an image capturing apparatus having a pair of focus detecting pixels partially arranged in a two-dimensional image sensor constituted by a plurality of image forming pixels. The pair of focus detecting pixels are configured to receive light from different areas of the exit pupil of the imaging lens through a light-shielding layer having an opening, thereby performing pupil division. This apparatus acquires imaging signals using image forming pixels arranged on the most part of the two-dimensional sensor and performs focus detection by obtaining an image shift amount from signals from the focus detecting pixels partially arranged on the sensor.
Consider, for example, a camera with interchangeable lenses. In this case, if the exit pupil distance of the imaging lens differs from the incident pupil distance of the image sensor, an increase in the image height at the image sensor will cause a pupil shift between the exit pupil of the imaging lens and the incident pupil of the image sensor. In addition, the positional shift between a microlens and divided photo-electric conversion portions or between a microlens and a light-shielding layer having an opening due to mass-production variations causes a pupil shift between the exit pupil of the imaging lens and the incident pupil of the image sensor. In focus detection using the imaging plane phase-difference method, as a pupil shift occurs, the asymmetry of the respective partial pupil areas having undergone pupil division increases, resulting in a deterioration in focus detection accuracy.
Japanese Patent Laid-Open No. 2009-15164 discloses a technique of coping with a pupil shift by arranging a plurality of focus detecting pixels shifted from each other by different shift amounts for positioning between microlenses and divided photo-electric conversion portions. This technique performs focus detection using the imaging plane phase-difference method by selecting focus detecting pixels which minimize the asymmetry of the respective partial pupil areas having undergone pupil division in accordance with the imaging lens and the image height for focus detection.
In an actual image sensor, however, since each pixel has a finite size, there is an upper limit on the shift amount by which divided photo-electric conversion portions or light-shielding layers having openings are shifted inside pixels. This makes it impossible to cope with a pupil shift at the peripheral image height of the image sensor, resulting in a deterioration in focus detection accuracy. This then limits the image height range in which the pupil division phase-difference method can perform focus detection.